J. Am. Chem. SOC. 1987, 109, 6937-6943 here the number of possible KekulC structures grows exponentially with chain length.23 Recent calculations show that long-chain polymers of this family should be significantly table,^^^^^ and recent experimental work, though not fully characterizing the products, indicates that wider polymers of this family may existz7 The last group of molecules in Table IV compares several fragments which can be cut from the graphite (infinite 2-dimensional hexagonal) lattice. Those fragments identified with an asterisk can be shown to provide upper bounds to the energy per site of graphite2*since they can be repeated to cover the whole lattice. The most stable fragments seem to be those having the highest proportion of corrugated (polyphenanthrene-like) edge and the least zigzag (polyacene-like) edge. This may be related to the way in which edge erosion occurs in graphite.26*29 The results for our two largest molecules, coronene and dibenzvg,op]naphthacene, give us a chance to compare two ‘‘rules of thumb” for estimating the stability of conjugated benzenoid hydrocarbons. One rule states simply that the maximum stability for molecules with a given number of carbon atoms is attained when the perimeter of the molecule is minimized (Le., when the H / C ratio is m i n i m i ~ e d ) . ~For ~ 24 carbon atoms the minimum (27) (a) Murakami, M.; Yoshimura, S. J . Chem. Soc., Chem. Commun. 1984, 1649. (b) Murakami, M.; Yoshimura, S. Mol. Cryst. Liq. Cryst. 1985, 118, 95. (28) Klein, D. J.; Alexander, S. A,; Seitz, W. A.; Schmalz, T. G.; and Hite, G. E. Theor. Chim. Acta 1986, 69, 393. (29) Note the important difference between oxidizing and reducing environments. See: (a) Wong, C.; Yang, R. T.; Halpern, B. L. J . Chem. Phys. 1983, 78, 3325. (b) Mims, C. A,; Chludzinski, J. J., Jr.; Pabst, J. K.; Baker, J . Catal. 1984, 88, 98. (c) Thomas, J. M. In Chemistry and Physics of
Carbon; Walker, P. L., Jr., Ed.; Dekker: New York, 1965; Vol. 1.
6937
perimeter is that of coronene. On the other hand, the ideas of Clar31 lead one to conclude that the most stable molecules will be those which are most fully “benzenoid” (Le., for which one can draw Clar structures with as many circles as possible). For 24 carbon atoms this rule predicts dibenzlfg,op]naphthacene (4 circles) to be more stable than coronene (3 circles).32 In this view the central hexagon of coronene is essentially a hole which does not contribute to the aromaticity. As can be seen from Tables I1 and IV, both the raw valence bond energies and the Dewar-type aromatic stabilization energies here favor the minimum perimeter rule. In fact, the computed VB energy of coronene currently provides the best rigorous upper bound to the VB energy of the graphite latticesz8
Acknowledgment. W e thank Florida State University for providing us with a grant of computer time on their Cyber 205. Special thanks also go to Dr. Kenneth Flurchick of ETA Systems, Inc. for his assistance with the coronene calculation. Registry No. Benzene, 7 1-43-2; naphthalene, 9 1-20-3; biphenyl, 9252-4; anthracene, 120-12-7; phenanthrene, 85-01-8; stilbene, 588-59-0; pyrene, 129-00-0; naphthacene, 92-24-0; benz[a]anthracene, 56-55-3; benzophenanthrene, 65777-08-4; chrysene, 2 18-01-9;triphenylene, 21759-4; p-terphenyi, 92-94-4; m-terphenyl, 92-06-8; o-terphenyl, 84-15-1; perylene, 198-55-0; benz[a]pyrene, 50-32-8; benz[e]pyrene, 192-97-2; pentacene, 135-48-8; picene, 213-46-7; benz[b,cd]pyrene, 191-26-4; benz [ghilperylene, 191-24-2; dibenzo [fg,op]naphthacene, 192-51-8; coronene. 191-07-1. (30) See, for example: Stein, S. E. J . Phys. Chem. 1978, 82, 566. (31) Clar, E. The Aromatic Sextet; John Wiley and Sons: London, 1972. (32) Dias, J. R. Acc. Chem. Res. 1985, 18, 241.
Cycloalkylmethyl Radicals. 5. 6- to 15-Membered Rings: EPR Studies of Ring Conformations and Stereodynamics’ K. U. Ingold** and J. C. W a l t ~ n * ~ Contribution from the Division of Chemistry, National Research Council of Canada, Ottawa, Ontario K I A OR6, Canada, and Department of Chemistry, The University, St. Andrews, Fife, Scotland K Y l 6 9ST, U.K. Received November 12, 1986
Abstract: Cycloalkylmethyl radicals having 7- to 15-membered rings (excluding cyclotetradecylmethyl) have been generated and examined by EPR spectroscopy. All the cycloalkylmethyl radicals from the 9-membered ring upward show two distinguishable conformers, though there may be additional conformers present with cyclodecylmethyl and cyclopentadecylmethyl radicals. All the cycloalkylmethyl radicals from the IO-membered ring upward have one conformer, the quasi-equatorial, QE, with an H, hfs in the range 27.7-32.0 G at 140 K and one conformer, the quasi-axial, QA, with an H, hfs in the range 38.3-40.4 G at 140 K. The QE and QA conformers have been assigned to species in which the CH2’ group occupies “outer-edge” and “cornern sites, respectively, in the preferred conformations of the cycloalkanes. The EPR results indicated that the preferred conformations of the C9, Cl0,CI1,Clz, CI3,and CISrings were [333], [2323], [335], [3333], [346], and [33333], respectively. Arrhenius parameters for ring-atom site exchange in the cycloalkylmethyl radicals have been determined from the exchange broadening in the EPR spectra. These EPR barriers are compared with literature data on cycloalkane free energy barriers measured by N M R and enthalpic barriers estimated by force field and related methods.
The C,H2’ group in cycloalkylmethyl radicals, 1, provides a novel and useful conformational “spin probe” that can be monitored by EPR spectro~copy.“~ This probe yields both the pre( 1 ) Issued as NRCC No. 27590. Part 4: Reference 4. (2) NRCC. (3) The University, St. Andrews. (4) Walton, J. C. J . Chem. Soc., Perkin Trans. 2 1986, 1641-1646. (5) Ingold, K . U.; Walton, J. C. J. Am. Chem. SOC.1985,107,6315-6317. (61 Ingold. K. U.: Walton. J. C. J . Chem. Soc.. Perkin Trans. 2 1986. 13 37- I 344. (7) Kemball, M. L.; Walton, J. C.; Ingold, K. U. J . Chem. Soc., Perkin Trans. 2 1982, 1017-1023.
ferred conformation of the C,H: group with respect to the C,-H, bond and, more interestingly, information regarding ring conformations and even, in suitable cases, information regarding the dynamics of interconversion of ring conformers. For example,
both the cYclohexYlmethYl radical and the cis-(4-methYlcYclohexy1)methyl radical exist as mixtures of two conformers, one
0002-7863/87/ 1509-6937$01.50/0 Published 1987 by the American Chemical Society
Ingold and Walton
6938 J . Am. Chem. SOC.,Vol. 109, No. 23, 1987
Table I. Hydrogen Hfs (in Gauss) of Cycloalkylmethyl Radicals at 140 K
ring size n 6
conformna axial, 3b equatorial, 4b
I 8
T/ K
having CHI' axial and one having CH2' e q ~ a t o r i a l . ~From ~~ quantitative measurements of the concentrations of the axial and equatorial conformers over a range of temperatures, the conformational enthalpic and entropic axial/equatorial preference of the CH2' group on a 6-membered ring was determined.5s6 Furthermore, for the cis-(4-methylcyclohexyl)methyl radical the dynamics of the cyclohexane ring inversion were determinedS6The CH,' group has proved equally useful in determining the conformational preferences and equilibria of the cyclohex-2-enylmethyl and cyclohex-3-enylmethyl radical^.^ N M R spectroscopy has, of course, been extremely successful in defining the conformational preferences and equilibria of cyclohexane and its derivatives but has proved to be less useful with larger rings. This is due to two factors. First, the barriers to conformational interconversion of the larger rings are considerably lower than the barriers in cyclohexanes. It is therefore difficult to Ufreeze-outnthe motions of larger rings on the N M R time scale. Second, the actual equilibria are complex, and if, in addition, the symmetry is low, then very complex N M R spectra are obtained. For these reasons, there have been comparatively few N M R studies of the larger cycloalkanes and the interpretation of the results has been difficult and not infrequently rather inconclusive. As we have previously r e p ~ r t e d ,the ~ cycloundecylmethyl radical, 1 (n = 1l ) , exists in two distinct forms at temperatures C230 K; these forms interconvert rapidly on the EPR time scale at higher temperatures. The main spectroscopic difference between these two radicals lies in the magnitudes of their H, hyperfine splittings (hfs); a t 140 K one radical has aHg = 38.3 G , and the other, aHg = 31.1 G. It therefore seemed probable to us that other 1 with n 3 6 might give EPR spectra showing the presence of more than one conformer. Such proved to be the case for 1 (n = 9-13, 15),8 as we report herein, but not for 1 (n = 7)9 nor for 1 (n = 8).' Our conclusions are compared with what little information is available on the larger cycloalkanes from studies by N M R , by X-ray diffraction, and by force field and other theoretical treatments.
Results The cycloalkylmethyl radicals, 1, were formed by bromine atom abstraction from the corresponding bromides with photochemically generated triethylsilyl or trimethyltin radicals in tert-butylbenzene a t T k 200 K and in cyclopropane or propane a t lower temperatures. The EPR parameters for the radicals observed at 140 K (which were confirmed by spectral simulation) are given in Table I. The H, hfs all showed negative temperature coefficients; i.e., a H g decreased with increasing temperature (see Figure 1 ) . This indicates that in all these radicals the preferred conformation about (8) And, of course, for n = 6 , see ref 5 and 6. We have not yet examined 1 (n = 14). (9) Walton, J. C., unpublished results.
uHg ( H )
uH (other)
22.1 22.2 22.1 21.6 22.3 22.3 22.3 22.3 21.4 21.4 22.0 22.0 22.2 22.2 22.0 22.0
42.4 30.7 37.6 40.1 37.2 -35.0 40.5 31.9 38.3 31.1 40.5 32.0 39.3 30.4 39.3 27.7
0.75 (5 H ) 0.95 (4 H)
0.8 (4 H) 0.8 (4 H ) QA QE 10 QA QE' 11 QA QE 12 QA QE 13 QA QE 15 QA QEC "QA = quasi-axial; QE = quasi-equatorial. bData are for the cis(4-methylcyc1ohexyl)methyl radical because the axial conformer of cyclohexylmethyl is present at too low a concentration to be detected at 140 K.536'There may be two QE conformers (see text). 9
Figure 1. Values of uHg as a function of temperature for 1. Key: n = 6, 0 ; n = 9, V; n = 10, A; n = 11, A; n = 12.W; n = 13, 0;n = 15, 0. The upper curves correspond to the quasi-axial, QA, and the lower to the quasi-equatorial, QE, radical conformers.
uH. (2 H)
the C,-C,' bond is one in which the C, 2p, orbital (Le., the SOMO) eclipses the C g H , bond, Le., 2.
/
\
' * (CH;), 2
We have shown p r e v i o ~ s l ythat ~ ~ ~two conformers of the cyclohexylmethyl radical can be observed: The one with the larger aHghas the CH2' group axial, and that with the smaller uHg has this group equatorial. The different magnitudes of the H, hfs arise because the barrier to rotation about the C,-C,' bond is considerably higher in the axial conformer, 3 (because of steric interactions between H, and H3, H5 during rotation), than in the equatorial conformer, 4. The axial radical, 3, therefore has a
3
4
stronger preference for conformation 2 than does the equatorial radical, 4 and this is reflected by a larger H, hfs because orbital overlap between the SOMO and the C,-H, bond is enhanced. The results of Table I are remarkable in that all cycloalkylmethyl radicals having n 3 9 are present in two and only two readily distinguishable conformations despite the fact that a considerable number of conformations are, in principle, possible for such large rings. Those 1 having n 3 10 have one conformer with a small H, hfs (27.7-32.0 G at 140 K) and one with a larger H, hfs (38.3-40.4 G at 140 K). By analogy with 1 (n = 6) we classify the smaller H, hfs conformers as quasi-equatorial, QE, and the larger H, hfs conformers as quasi-axial, QA. Furthermore, we conclude that the barrier to rotation of the CH2' group is greater in the QA than in the Q E conformation. 1 (n = 6-8). Our studies on 1 (n = 6) and on cis- and trans-(4-alkylcyclohexyl)methylradicals have already been reported in some detail.5$6 We include data on 1 (n = 6) merely for comparative purposes. The same is true for 1 (n = 7).9 We previously reported that 1 (n = 8) exists as only one conformer.' In view of our present results with larger rings, this seemed rather
J . Am. Chem. SOC.,Vol. 109, No. 23, 1987 6939
Cycloalkylmethyl Radicals
Table 11. Van’t Hoff Parameters for the QA and QE Equilibriau
ring size n
Gibbsimol
6 9 10 11 12 13 15
3.6 f 0.5 2.9 f 0.5 4.0 f 0.2 0.1 f 0.03 -0.2 f 0.03 0.1 f 0.02 -2.5 f 0.1
A
s
o I
AH’ kcalimol ~
1.79 0.32 0.56 0.09 0.14 0.31 -0.58
f 0.04 f 0.02 f 0.06
f 0.01 f 0.02 f 0.02 f 0.05
AG0300K.
kcal/mol -0.71 0.55 0.64 -0.06 -0.20 -0.28 -0.17
“Calculated from the [QA]:[QE] ratios presented in Figure 3. The data for 1 (n = 6) have been given p r e v i o ~ s l y . ~ ~ ~
260K
-
J
11
/%ft 5‘b4+wM,uT IOG
Figure 2. EPR spectra (low-field halves, 9.4 GHz) of 1 (n = 9) in cyclopropane a t various temperatures. The inset shows the central line (mp = -I/*, m, = 0) a t high resolution as a second-derivative presentation. Q A and Q E refer to quasi-axial and quasi-equatorial conformers, respectively.
O
5r 295
-.-,~ -\,-:
. - IOG
ir
.
,*.
5
.
Figure 4. EPR spectra (low-field halves, 9.4 GHz) of 1 ( n = I O ) in tert-butylbenzene a t various temperatures.
that the quasi-equatorial, QE, conformer predominates for T < 110 K and the quasi-axial, QA, conformer at higher temperatures. At 150 K the Q A conformer shows fine structure with hfs by four equivalent y-hydrogens (see inset in Figure 2). No fine structure could be resolved in the QE conformer’s spectrum nor in the spectrum of any 1 (n > 9). The van’t Hoff parameters for the Q A s Q E equilibrium, derived from the variation in the [QA]/[QE] ratio with temperature, are given in Table 11. Error limits are rather large because of relatively poor resolution of the Q A and QE spectra. The E P R spectra showed line broadening between 180 and 260 K, sharpening at the latter temperature to tgive a single averaged spectrum with uH=(2 H ) = 22.2 G and uHg 2 \ * - I 5 2I (1 H ) = 33.3 G (see Figure 2). The exchange-broadened spectra 3 4 5 6 7 8 9 IO I1 were ~ i m u l a t e d with ’ ~ a two-jump model, the relative concenlo3 KIT trations of Q A and Q E being obtained by extrapolation of the Figure 3. Plots of log ([QA]/[QE]) vs 1 0 3 K / T for 1. Key: n = 6, 0 ; low-temperature data. Arrhenius parameters for the forward and n = 9, V; n = 10, A; n = 11, A; n = 12, B; n = 13, 0;n = 15, 0. reverse processes, defined in eq 1, are given in Table 111. These surprising, particularly since the barrier to ring inversion of cyclooctane is ca. 7-8 kcal/mol, according to NMR measurements.’Ox’l Such a barrier should make the slow-exchange limit parameters are less precise than we would wish because rather for conformational interconversion of cyclooctylmethyl radicals poor spectral resolution produces a certain “softness” in the fits readily accessible in the temperature range of our E P R studies. of calculated and experimental spectra. Nevertheless, upon reexamining this radical, we could still detect 1 ( n = 10). The Q A and Q E radicals have well-resolved spectra only a single conformer in the temperature range 110-270 K. (Figure 4), and a plot of log ([QA]/[QE]) vs 1 / T is shown in From the signal/noise ratio a t 210 K we estimate that the conFigure 3, with the derived van’t Hoff parameters given in Table centration of any other conformer must be 110 (see Figure 3). The line broadening observed in the E P R spectra of cyclononylmethyl results from the exchange of corner and outer-edge species, Le., from the interconversion of 5 and 6. Dale25has shown that interconversion in cyclononane takes place by a “corner migration” via a barrier of the [1233] type to give cyclononane in the [234] conformation.26 The [234] conformer is then con(17) This conformation is referred to as a “twist boat-chair” by many workers. (18) (a) For a comprehensive listing of force field calculations on cyclononane see ref 19. (b) This reference also provides an excellent summary of the results obtained by force field calculations, NMR, etc., on the conformations of other cycloalkanes. (19) Burkert, U.; Allinger, N. L. Molecular Mechanics; ACS Monograph 177; American Chemical Society: Washington, DC, 1982; pp 89-108. (20) The most recent calculations on cyclononane2’ give the following as strain energies (in kcal/mol) relative to the [333] conformation. Energy minima as follows: [333], 0; [144], 1.2; [225], 1.7; [234], 3.2. Conformational transition states as follows: [1323], 4.4; [I2241 6.0; [1233], 7.0; [ 1 8 ] , 8.3; [1134], 9.9. (21) Anet, F. A. L.; Krane, J. Isr. J . Ckem. 1980, 20, 72-83. (22) Anet, F. A. L.; Wagner, J. J . J . A m . Ckem. SOC.1971, 93, 5266-5268. (23) Dale, J. Acta Ckem. Scand. 1973, 27, 1149-1 158. (24) Eliel, E. L . J . Ckem. Educ. 1971, 48, 163-167; 1980, 57, 52-55.
Because the corner C-H bonds are at equal angles with respect to the plane of the ring, the word “isoclinal” has also been used to describe them. See: Hendrickson, J. B. J . Am. Ckem. SOC.1964, 86, 4854-4866. (25) Dale, J. Acta Ckem. Scand. 1973, 27, 1130-1 148. (26) The notation for energy minima’5,’6is expanded25 to barriers by defining the syn eclipsed bond as a one-bond side; it is marked in italics.
J. Am. Chem.SOC.,Vol. 109,No. 23, 1987 6941 verted to its mirror image via a [1323] barrier and hence via a second [I2331 barrier back to the [333] conformer, in which all the carbons have moved one step around the ring, i.e., 5 6. Repetition of this cycle interchanges all ring sites in cyclononane. However, the CH2’ group in the cyclononylmethyl radical cannot adopt inner-edge sites, and so the radical cannot undergo the complete cycle. Interconversion of the CHI’ group occurs by partial cycles ([333] z [1233]* 2 [234] 2 [1323]* z [234] s [1233]* s [333]) as shown in Scheme I. Calculations suggest that the [1233]* transition state is the highest energy point on this cycle.20,21,25,27 1 ( n = 10). X-ray diffraction studies have shown that the rectangular [2323] conformation (see 7-9) predominates in crystals of several derivatives of c y c l o d e ~ a n e .The ~ ~ ~preference ~~
-
7 , C23231
8,[ 2 3 2 31
9, [2323]
for this conformation has also been confirmed by electron diffraction.30 The slow-inversion-limit ‘H and I3C N M R spectra could not be obtained for c y ~ l o d e c a n e , ~but ’ the I9F N M R spectrum of 1,l-difluorocyclodecane is consistent with the [2323] c ~ n f o r m a t i o n . ~Modern ~ force field calculations have also pinpointed [2323] as the lowest enthalpy cyclodecane conformer. 18b,19,30,33,34 Three cyclodecylmethyl conformers would be expected (7-9). Rotation of the CH2’ group will be most strongly impeded in the corner conformer, 7 (again by a 1,3-interaction with the H atom on C(3) of the ring). W e therefore identify 7 as the quasi-axial radical with the large H, hfs. When the CH,’ group is on the “short side”, i.e. 9, there will be negligible hindrance to its rotation (the H atoms on C(3) are ”round the corner” and about as far removed from CH2’ as is possible). Models suggest that when the CH,’ group is on the “long side”, i.e. 8, there will be only minor steric interaction with the H atom on C(3). W e therefore identify both 8 and 9 as giving E P R spectra of the quasi-equatorial type. However, 8 will probably have a slightly larger H, hfs than 9, which provides a rationale for our partial resolution of two Q E radicals.35 On statistical grounds the ratio of concentrations of 7:8:9would be 8:4:2, i.e., QA:QE 8:6. Experimentally the Q A conformers predominate a t T k 140 K and the Q E conformers predominate a t T 5 140 K (see Figure 3). 1 ( n = 11). Force field and other calculations have indicated that several cycloundecane conformations have rather similar strain e n e r g i e ~ . ’ ~ ~ ’The ~ ~ ~most ’ ~ .recent ~ ~ ~ ~ ~calculation^^^ suggest that the triangular [335] conformer, 10, may be preferred, although two quinquangular conformers, [ 123231 and [ 123141, and another triangular conformer, [344], are close in energy. An X-ray 10 as the prediffraction study of c y c l o ~ n d e c a n o n esupported ~~ ferred conformer. The N M R spectra of cycloundecane are also
-
(27) Rustad, S.; Seip, H. M. Acta Ckem. Scand., Ser. A 1975, A29, 378-380. (28) Dunitz, J. D. Pure Appl. Ckem. 1971, 25, 495-508. (29) This conformation has also been referred to as the boat-chair-boat. (30) Hilderbrandt, R. L.; Wieser, J. D.; Montgomery, L. K. J . Am. Ckem. SOC.1973, 95, 8598-8605. (31) Anet, F. A. L.; Cheng, A. K.; Wagner, J. J. J . Am. Ckem. Sor. 1972, 94, 9250-9252. (32) Noe, E. A.; Roberts, J. D. J . Am. Ckem. SOC.1972, 94,2020-2026. (33) Allinger, N. L. J . Am. Ckem. SOC.1977, 99, 8127-8134. (34) White, D. N.; Bovill, M. J . J . Chem. Soc., Perkin Trans. 2 1977, 1610-1623. (35) Since calculations suggest that there is at least one other cyclodecane conformation, [I3241 (sTCCC), rather close in energy to the [2323] con-
f~rmation,’~ we cannot rule out the possibility that the second QE radical does not belong to a [2323] structure. However, the low temperatures of our experiments, viz., -145 to -89 ‘C, make this rather unlikely. (36) Engler, E. M.; Andose, J. D.; Schleyer, P. v. R . J . Am. Chem. SOC. 1973, 95, 8005-8025. (37) Anet, F. A. L.; Rawdah, T. N . J . Am. Chem. Soc. 1978. 100, 7810-78 14. (38) Groth, P. Acta Ckem. Scand., Ser. A 1974, 28, 294-298.
6942 J. Am. Chem. Soc.. Vol. 109, No. 23, 1987
Ingold and Walton Scheme I1
IO. E3351
consistent, with 10 being the main species present at low temperature~.~~,~~ The EPR spectral data are readily rationalized in terms of conformer 10, which would give two distinguishable cycloundecylmethyl radicals: a corner type, which we identify with the QA spectrum (larger H, hfs), and an outer-edge type, which we identify with the Q E spectrum. The statistical ratio of corner to outer-edge sites is 6:8, which is close to the experimental Q A Q E ratios (e.g., 0.79 a t 140 K; see also Figure 3). Corner and edge positions are interchanged by a type of pseudorotation in cyclou n d e ~ a n e , but ~ ~ complete .~~ pseudorotation is unlikely in the cycloundecylmethyl radical because the CHI' group is unlikely to be able to occupy an inner-edge site. Nevertheless, corner and outer-edge radicals can exchange by partial pseudorotations. 1 ( n = 12). The lowest energy conformation of cyclododecane has the symmetrical, quadrangular [3333] structure, 11 and 12, of D, symmetry (Scheme 11). This "square" conformation has been found in the crystalline state,39is consistent with N M R data,31.40and is the one calculated to be of lowest enthalpy by various methods.'5J8b-19~36~40 Other conformations are higher in enthalpy1s,40and probably will not contribute in the accessible temperature range. The two cyclododecylmethyl radicals are of the corner (homotopic) type, 11, and outer-edge (equatorial) type, 12, which we identify as Q A and QE, respectively. On statistical grounds, the [11]:[12] ratio would be 1.0; the experimental ratio is 0.6 a t 150 K (see Figure 2). The only possible sequence for the exchange of corner and edge position^^^^^^ that avoids placing the CH2' group in an inner site is shown in Scheme 11. 1 ( n = 13). There are five low-enthalpy conformations of c y c l o t r i d e ~ a n e ,with ~ ~ ~ two ~ ~ ~quinquangular ~~ structures, 13 [13333] and 14 ([12433], and a trinagular, 15 [346], being of lowest energy.41 The 13CN M R spectrum of cyclotridecane defied a n a l y ~ i s , ~but ' an X-ray structure of a 13-membered ring containing nitrogen showed that the main conformer had the [ 133331 str~cture.~'
13, [I33331
14, [I24331
15. E3461
Both the quinquangular conformations have 10 corner sites and 8 outer-edge sites for the CH2' group, so the corner to edge statistical ratio is 1.25. This ratio is very different from the experimental [QA]:[QE] ratio, e.g., 0.37 at 150 K (see Figure 3). Models indicate that the corner sites next to the one-bond edge are not homotopic. The CHI' group in these two corners can be in either a pseudoaxial or pseudoequatorial position. In the former position, there would be significant steric hindrance (1,3-interaction) to rotation of CHI', but in the latter position, hindrance would be minimal. These particular corner radicals should therefore show distinct EPR spectra, but no "extra" lines could be detected and line widths were normal. While this does not exclude conformations 13 and 14, since low concentrations of these novel corner radicals might escape detection, it is obvious that our EPR data are in better accord with cyclotridecylmethyl ~~~~~~~
~
11, [33331
[2334]
E23341
12, [33331
Scheme 111
16
radicals existing mainly in the triangular conformation, 15. For this conformer the corner to outer-edge statistical ratio is 0.6, which is much closer to the observed [QA]:[QE] ratios (see Figure 3). 1 ( n = 15). Calculations indicate that cyclopentadecane has five low-enthalpy conformations, all of the quinquangular type.15,37 The [33333] conformer is lowest in enthalpy's,37but is entropically di~favored'~ relative to the other four, less symmetric, low-enthalpy conformers, viz., [13443], [13434], [14334], and [13353], and for this reason cyclopentadecane probably exists as a mixture of all five conformers. For any quinquangular cyclopentadecane the statistical corner to outer-edge ratio is 1.0, which is not too different from the experimental [QA]:[QE] ratios (e.g., 2.0 a t 150 K, see Figure 2). The four low-energy, one-bond-sided conformers should give rise to novel corner radicals as indicated above for 1 ( n = 13). The unusually broad lines of the Q E radical a t the slow-exchangelimit might be interpreted as favoring a contribution from these novel corner radicals. Dynamics of Corner to Outer-Edge Interconversion of Cycloalkylmethyl Radicals. The calculated Arrhenius parameters for the QA QE, corner to outer-edge, interconversions and for the Q E 9 QA, outer-edge to corner, interconversions are given in Table 111. The preexponential factors are not sufficiently accurate to warrant detailed discussion. We therefore note only that they are all reasonably close to the expected value of l O I 3 s-'. The activation energies of the forward reactions and back-reactions differ because the corner and outer-edge radicals differ in enthalpy. The mean of Efand Eb should be close to the activation energy for ring-atom site exchange in the unsubstituted cycloalkane because a single, small substituent will have little or no effect on these barriers.",23 In the outer edge to corner migration of the CH2' group, the transition state, 16, has one torsional angle equal to 0' and the (see Scheme 111). It therefore other two equal to 120° resembles the syn barrier for the n-butane internal rotation about the central C-C bond, which has a value of ca. 6 or 7 kcal/ mo1.15336342Values of Efand Eb for 1 ( n 3 9) are of this general magnitude (see Table 111). This is somewhat surprising, as enhanced barriers in the rings might be expected, since the axial hydrogens on C(2) and C(5), H,cannot avoid each other in the transition state, 16, as efficiently in rings as in n-butane. Perhaps this anticipated steric enhancement of the ring barriers is offset by an increase in the C-C-C angles in the rings. 25337
~
(39) Dunitz, J. D.; Sheaver, H. M. M. Hela. Chim. Acta 1960,43, 18-35. Dunitz, J. D. Perspect. Struct. Chem. 1968, 2, 1-70. (40)Anet, F. A. L.; Rawdah, T. N . J . Am. Chem. SOC. 1978, 100, 7166-7 171. (41)Rubin, B. H.; Williamson, M.; Takeshita, M.; Menger, F. M.; Anet, F. A. L.; Bacon, B.; Allinger, N. L. J. Am. Chem. SOC.1984,106,2088-2092.
(42) Ita, K.J . A m . Chem. SOC.1953,75, 243C-2435. Piercy, J. E.;Rao, M. G. S . J . Chem. Phys. 1967,46, 3951-3959. Hoyland, J. R. Ibid. 1968, 49, 2563-2566. Radom, L.;Pople, J. A. J . Am. Chem. SOC.1970, 92, 4786-4795. Radom, L.;Lathan, W. A,; Hehre, W. J.; Pople, J A Ibid. 1973, 95, 643-698.
Cycloalkylmethyl Radicals Since the entropies of activation will probably be small, the free energies of activation, AG*, estimated from the I3C N M R coalescence temperatures of the cycloalkanes would be expected to be similar to the mean of our values for Efand Eb. Such appears to be the case (see Table 111). This table also includes the enthalpic barriers estimated by DaleZSand barriers calculated by force field methods. Dale's values reproduce the trends we observe experimentally but are generally too high, probably for the reasons he discusses.25 Our measurements agree reasonably well with the available barriers calculated by force field methods.
Experimental Section ' H and "C N M R spectra were recorded on a Bruker W P 80 and a Varian CFT 20 instrument, respectively, in CDCI, at ambient temperature with tetramethylsilane as internal standard. Mass spectra were obtained on an A.E.I. MS 902 spectrometer. EPR spectra were recorded on a Bruker ER 200 D instrument with samples that had been degassed by several freeze-pump-thaw cycles. They were sealed in 4-mm-0.d. Spectrosil tubes and irradiated in the cavity of the spectrometer with light from a 500-W super-pressure mercury arc lamp. Methylenecyclononane. To n-butyllithium (65 mL, 1.6 M in hexane) in dry ether (100 mL) under N2 was added methyltriphenylphosphonium bromide (23 g), and the solution was stirred for 60 min. Cyclononanone (6 g) in ether (30 mL) was added dropwise, and the solution was then refluxed for 10 h. Ether and hexane were distilled off, and glyme (100 mL) was added. The mixture was heated to ca. 85 OC for 4.5 h, cooled in ice, and filtered and the glyme solution extracted with light petroleum ether (5 X 50 mL). The petroleum extracts were washed with water (3 X 30 mL), dried over Na2S04, and distilled to give methylenecyclononane: bp 110 "C (20 Torr); 54% yield; ' H N M R 6 1.3-1.9 (6 H, br m), 1.9-2.3 (2 H , br m), 4.85 (2 H , s). Cyclononylmethanol. To methylenecyclononane (3 g) in dry T H F (40 mL) cooled in ice was added BH3.THF complex (16 mL). The solution was stirred 30 min, and water (2.3 mL) was added; the mixture was then heated at 45 O C , and 3 M N a O H (12 mL) was added, followed by H 2 0 2 (5.5 mL of a 30% solution). This mixture was stirred for 60 min and cooled, ether (100 mL) was added, and the mixture was then separated. The aqueous layer was saturated with NaCl and extracted twice with ether. The combined ether extracts were dried and distilled to give cyclononylmethanol: bp -180 "C (20 Torr); 86% yield; 'H N M R 6 1.52 (18 H , br s), 3.42 (2 H , d, J = 6 Hz). Cyclononylmethyl Bromide. Cyclononylmethanol (2.9 g) and triethylamine (1.9 g) in dichloromethane (120 mL) were cooled in ice, and CH3S02CI (2.3 g) was added, drop by drop. After the mixture was stirred for 30 min, water was added and the CH,CI, layer was separated, washed with 2 M HCI, brine, and N a H C 0 3 solution, and dried over Na,SO,. The CH,C12 was then removed on a rotary evaporator at room temperature. The mesylate was added to LiBr (5.0 g) in dry acetone (65 mL), this mixture was refluxed for 18 h and filtered, and the acetone was removed on a rotary evaporator. The residue was dissolved in light petroleum ether, which was washed with water, dried (Na2S04),and then chromatographed on a column of silica gel. The bromide was eluted with the first 200 mL of light petroleum ether. It was distilled: bp 78 O C (0.5 Torr); 62% yield; ' H N M R 6 1.5 1 (1 7 H, br s), 3.62 (2 H, d, J = 6 Hz); "C N M R 6 23.7, 24.7, 25.2, 30.0, 38.4, 41.8; M+(obsd) = 218.0659, C l o H 1 919Br requires 218.0670.
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J. Am. Chem. SOC.,Vol. 109, No. 23, 1987 6943 Cyclodecylmethyl Bromide. This was prepared from cyclodecanone by the same method as cyclononylmethyl bromide and was purified in the same way: bp -100 'C (0.5 Torr); 83% yield; ' H N M R 6 1.6 (18 H , s), 1.9 (1 H , m), 3.5 (2 H , d, J = 6 Hz); M+(obsd) = 234 and 232. Cycloundecylmethyl Bromide. This was prepared as described previously.7 Methylenecyclododecane. To a suspension of zinc dust (14.8 g) in CH2Br2 (13.0 g) and dry T H F (250 mL) was added 55 mL of a 1.0 M solution of TiCI, in di~hloromethane.~'The suspension was stirred for 15 min, and then cyclododecanone (9.1 g) in T H F (50 mL) was added dropwise and the mixture stirred overnight. The mixture was diluted with ether, and the organic layer was washed with 1 M HCI and brine, then dried (Na2S04),and chromatographed on alumina after removal of the solvent. The product was distilled: bp 76 O C (0.6 Torr); 70% yield; 'H N M R 6 1.4 (18 H , br s), 1.9-2.2 (4 H , m), 4.80 (2 H, s). Cyclododecylmethyl Bromide. A rapid stream of dry HBr was bubbled through a solution of methylenecyclododecane (2.0 g) and benzoyl peroxide (0.1 g) in light petroleum ether (16 mL) for 30 min at 20 O C . Nitrogen gas was then passed through the solution for 15 min to remove unreacted HBr. The mixture was diluted with light petroleum ether (40 mL), washed with N a H C 0 3 solution, and dried, and the solvent was removed. Crystals of benzoic acid, which separated on standing overnight, were also removed, and the residual oil was distilled: bp 120-122 O C (5 Torr); 90% yield; ' H N M R 6 1.4 (23 H , br s), 3.45 (2 H , d, J = 6 Hz); I3C N M R , unresolved group of peaks at 6 20-26, plus singlets at 6 28.2, 36.7, and 40.0; M+(obsd) = 262 and 260. Cyclotridecylmethyl Bromide. Methylenecyclotridecane was prepared from the ketone by the Wittig procedure described above and was converted to the bromide by HBr addition under free-radical conditions, again as described above: bp 92-94 O C ( Torr); 62% yield; ' H N M R 6 1.35 (25 H , br s), 3.35 (2 H , d, J = 6 Hz); Mt(obsd) = 276 and 274. Cyclopentadecylmethyl Bromide. This was prepared from the ketone by the Wittig reaction, followed by HBr addition to the alkene: bp 120-122 O C (lo-' Torr); 75% yield; ' H N M R 6 1.3 (29 H, br s), 3.30 (2 H , d, J = 6 Hz); M*(obsd) = 304 and 302.
Acknowledgment. We thank NATO for the award of a research grant, without which the present work would not have been undertaken. J.C.W. thanks the Carnegie Trust for a travel grant. We also thank Dr. I. MacInnes, S. Blake, and L. P. Henry for carrying out some of the experimental work. Registry No. 1 (n = 6), 67271-34-5; 1 (n = 7), 71880-18-7; 1 (n = 8), 83386-44-1; 1 (n = 9), 110569-64-7; 1 (n = lo), 110569-65-8; 1 (n = 1l ) , 83386-45-2; 1 (n = 12), 110569-66-9; 1 (n = 13), 110569-67-0; 1 (n = 15), 110569-68-1.
Supplementary Material Available: Table I V giving conformer ratios a t various temperatures for 1 (n = 6, 9-1 3, 15) (1 page). Ordering information is given on any current masthead page. (43) Takai, K.; Hotta, Y.;Oshima, K.; Nozaki, H. Tetrahedron Leu. 1978, 2417-2420. (44) Wiberg, K. B.; Boyd, R. H. J . Am. Chem. SOC.1972,94,8426-8430. (45) Anet, F. A. L.; Cheng, A. K., unpublished results cited in: Dale, J. Top. Stereochem. 1976, 9, 199-270.
